Mercury emissions from coal-fired boilers and from other combustion processes, including municipal waste combustors, gold ore roasters, and hazardous waste incinerators have been studied to determine emissions rates and effectiveness of control technologies. Factors that differ between coal-fired boilers and ore roasters may include mercury content of the feed and flue gas, flue gas sulfur dioxide concentration, and flue gas mercury speciation. Analyses of untreated flue gas from coal-fired boilers have detected mercury concentrations up to 25 ug/m3, with an average concentration of 15 ug/m3. Gold ores have measured mercury concentrations from less than 1 mg/kg to 200 mg/kg for ore deposits in the western U.S. It is thus desirable, and in many countries it may be legally required, to remove contaminating mercury from off gases before these are released to the environment.
Studies of treatment technologies for mercury removal from coal-fired boiler emissions have shown that speciation of mercury may impact mercury removals. Mercury species in combustion gases may include elemental mercury vapor (Hg0), oxidized mercury vapor (Hg+2) and mercury associated with particulate matter (HgP). Elemental mercury may be a dominant species in roaster flue gas.
It is thought that the mercury in off gases from ore roasters and the like may be present as water-insoluble elemental mercury (Hg) and water-soluble ionized mercury (Hg2+). Although ionized mercury, which is easily absorbed by water, can be removed by a desulfurization absorption tower or the like, elemental mercury, which has a very low solubility in water, may not be absorbed by a desulfurization absorption tower, and may be discharged as elemental mercury vapor without being absorbed.
Conventionally, two major approaches have been proposed for the removal of mercury from gases: an activated carbon adsorption method and a mercuric chloride absorption method in which elemental mercury is converted to mercurous chloride by the reaction with mercuric chloride. However in the foregoing conventional mercuric chloride method, when the mercury having been oxidized once is trapped by a downstream scrubber, it may be reduced by the action of SO2 coexisting in the flue gas, and may be volatized again. The effectiveness of the conventional mercury removal process may be limited by the reaction rate of the mercuric chloride with the elemental mercury.
A variety of methods and apparatuses for the removal of mercury from contaminated gases are well known in the prior art.
U.S. Pat. No. 5,409,522, Apr. 25, 1995 to Durham et al. discloses an apparatus and process for removing particular material and mercury-containing compounds from a gas stream by regenerating the sorbent used to recover the mercury.
U.S. Pat. No. 6,920,329, Nov. 1, 2005 to Sellakumar discloses an apparatus and process for converting mercury to mercuric chloride comprising contacting the mercury with a solution of a chloride salt and heating the solution/